Cylindrical rolls are utilized in a number of industrial applications, especially those relating to papermaking. Such rolls are typically employed in demanding environments in which they can be exposed to high dynamic loads and temperatures and aggressive or corrosive chemical agents. As an example, in a typical paper mill, rolls are used not only for transporting a fibrous web sheet between processing stations, but also, in the case of press section and calender rolls, for processing the web sheet itself into paper.
A papermaking machine may include one or more suction rolls placed at various positions within the machine to draw moisture from a belt (such as a press felt) and/or the fiber web. Each suction roll is typically constructed from a metallic shell covered by a polymeric cover with a plurality of holes extending radially therethrough. Vacuum pressure is applied with a suction box located in the interior of the suction roll shell. Water is drawn into the radially-extending holes and is either propelled centrifugally from the holes after they pass out of the suction zone or transported from the interior of the suction roll shell through appropriate fluid conduits or piping. The holes are typically formed in a grid-like pattern by a multi-bit drill that forms a line of multiple holes at once (for example, the drill may form fifty aligned holes at once). In many grid patterns, the holes are arranged such that rows and columns of holes are at an oblique angle to the longitudinal axis of the roll.
As the paper web is conveyed through a papermaking machine, it can be very important to understand the pressure profile experienced by the paper web. Variations in pressure can impact the amount of water drained from the web, which can affect the ultimate sheet moisture content, thickness, and other properties. The magnitude of pressure applied with a suction roll can, therefore, impact the quality of paper produced with the paper machine.
Other properties of a suction roll can also be important. For example, the stress and strain experienced by the roll cover in the cross machine direction can provide information about the durability and dimensional stability of the cover. In addition, the temperature profile of the roll can assist in identifying potential problem areas of the cover.
It is known to include pressure and/or temperature sensors in the cover of an industrial roll. For example, U.S. Pat. No. 5,699,729 to Moschel et al. describes a roll with a helically-disposed fiber that includes a plurality of pressure sensors embedded in the polymeric cover of the roll. However, a suction roll of the type described above presents technical challenges that a conventional roll does not. For example, suction roll hole patterns are ordinarily designed with sufficient density that some of the holes would overlie portions of the sensors. Conventionally, the sensors and accompanying signal carrier (e.g., a fiber or cable) are applied to the metallic shell prior to the application of the polymeric cover, and the suction holes are drilled after the application and curing of the cover. Thus, drilling holes in the cover in a conventional manner would almost certainly damage the sensors, and may well damage the signal carrier. Also, during curing of the cover often the polymeric material shifts slightly on the core, and in turn may shift the positions of the signal carrier and sensors; thus, it is not always possible to determine precisely the position of the signal carrier and sensors beneath the cover, and the shifting core may move a sensor or signal carrier to a position directly beneath a hole. Further, ordinarily optical fiber has a relative high minimum bending radius for suitable performance; thus, if optical fiber is employed as the signal carrier, trying to weave an optical fiber between prospective holes in the roll may result in unacceptable optical transmission within the fiber.
One approach to the use of sensors in a suction roll is described in U.S. Pat. No. 6,981,935 to Gustafson, which proposes that the signal carrier trace a path that follows the oblique angle of the suction roll drill pattern. This arrangement enables the signal carrier to be applied to the base layer of the roll cover prior to the application of the outer layer cover but still avoid damage to the signal carrier during drilling of the drainage holes. In some embodiments the sensor may be sufficiently large that it does not fit within the spaces between drainage holes. In such instances, a blind drilled hole may be formed in the cover over the sensor rather than a through hole so that the hole pattern in the cover is not disturbed. However, this solution may not be optimal for all roll covers. One approach utilizes sensors with apertures, with the drainage holes aligned with the apertures (see U.S. Pat. No. 7,572,214 to Gustafson (Gustafson), the disclosure of which is hereby incorporated herein in its entirety). Although this approach has promise, modifications that improve the design may be desirable.